1. Field of the Invention
The invention relates in general to a luminescent device, and more particularly, to an organic electroluminescent device.
2. Related Art of the Invention
The organic electroluminescent device having the characteristics of self-illumination, wide viewing angle (up to 160xc2x0), high response speed, low driving voltage, and full color has been practically applied to color flat panel displays such as a small area display panel, outdoor display panel, computer and television screen. The organic electroluminescent device uses two electrodes sandwiching an organic film with a luminescent property. When a direct voltage is applied to the organic electroluminescent device, holes are injected from the anode, while electrons are injected from the cathode. Due to the potential caused by the external electric field, carriers are mobile and recombine. A part of the energy released by the recombination of the electron and hole stimulates the luminescent molecules to a single excited state. When the excited luminescent molecules release energy and return to the ground state, a certain proportion of the energy is released to generate photons, which produce illumination. This is the mechanism for electroluminescence.
The basic structure of the conventional organic electroluminescent device includes a glass substrate, on which an emitting layer (EML) made of organic chemical is sandwiched by an indium-tin-oxide (ITO) anode and a cathode. To achieve the equilibrium between carrier injection and mobility, the organic electroluminescent device further incorporates a hole transport layer (HTL) between the emitting layer and the anode and an electron transport layer (ETL) between the emitting layer and the cathode, to inject and transport holes and electrons from the electrode, respectively, and a hole injecting layer (HIL) between the hole transport layer and the anode to inject holes into the emitting layer even when the voltage is low.
The currently available hole injecting layer is often selected from copper phthalocyanine (CuPc) material. FIG. 1 shows the graph of current density (mA/cm2) versus driving voltage (V) of organic electroluminescent devices A and B using CuPc/NPB and CuPc/2T-NATA as the hole injecting/transport layer, respectively. As shown in FIG. 1, when the material with high mobility is used as the hole transport layer, the hole injection rate is significantly increased; and consequently, the driving voltage is reduced. However, when material with high mobility such as 2T-NATA is used for the hole transport layer, the recombination of the carrier is inefficient.
The present invention provides an organic electroluminescent device of which the hole injection rate and the luminescent efficiency are both enhanced.
The organic electroluminescent device includes a cathode, an anode, an emitting layer, a hole transport layer, an electron transport layer, a hole injecting layer and an intermediate layer. The emitting layer is formed between the cathode and the anode. The hole transport layer is formed between the emitting layer and the anode. The electron transport layer is formed between the emitting layer and the cathode. The intermediate layer with high mobility is formed between the hole transport layer and the hole injection layer. The mobility of the intermediate layer is higher than that of the hole transportation layer and the hole injection layer.
The present invention incorporates an intermediate layer with relatively high carrier mobility between the hole transport layer and the hole injection layer, such that the hole injection rate is increased while the luminescent efficiency of the organic electroluminescent device is enhanced.